(1) Domain of the Invention
The invention relates, in a general manner, to the recognition of the instantaneous position of several mechanical parts capable of being displaced linearly by increments by means of a stepping motor.
More particularly, the invention relates, very specifically, to franking machines, which comprise value placing racks, arranged on the printing drum.
It is important to ensure that the actual position of the racks, on which the printed value will depend, is indeed the chosen position.
(2) Prior Art
In the case of franking machines where the value placing is carried out manually with the aid of thumb wheels (cf. the document U.S. Pat. No. 4,090,063), the risk of error is not high; this recognition problem can be resolved with the aid of magnetic sensors surrounding the printing drum or its shaft and effecting the detection during a revolution of the said drum; the printing order is not validated unless the recognition is positive.
In the more sophisticated franking machines of the type which the document U.S. Pat. No. 4,723,486 describes, the teaching of which is included here by reference, the value setting is carried out with the aid of a keyboard. It is then necessary to have an interface between the keyboard and the racks; this interface consists of a stepping motor allowing choice of the rack (selection) and its mechanical translation to a position dependent on the digit, from 0 to 9, chosen at the keyboard for the decade concerned (indexing).
In such a machine, where more relatively indefinite elements take part in the chain linking the placing of the new value by the operator and the positions actually taken by the racks, the problem of a recognition, of a verification of the positions of the racks arises with increased insistence. Moreover, the probability of an incident makes it desirable to carry out this recognition with minimal delay after the value setting and, in any case, before the setting in rotation of the printing drum. Because of this, in a machine of this type it is not possible to use simple magnetic sensors whose detection principle is generally based on the displacement of the objects to be detected
In the document U.S. Pat. No. 4,224,603 an effective absolute registering of the racks has been proposed. However, this registering requires, for each rack, the presence of several proximity sensors, adjacent to the rack concerned. The sensors and their cabling as a whole thus occupy a not insignificant space above the racks, which space it would be pleasing to be able to reduce.
Furthermore, by virtue of the document GB-A No. 2,194,842, a device allowing the registering of a single rack by virtue of a set of slits placed in the path of two luminous beams is known. Since two beam emitters and two beam receivers are necessarily required on either side of the rack concerned, it can be imagined that the adaptation of such a system to a four rack machine, as is most often the case, will take up a large volume, either in width or in height, according to the device adopted.
Finally, we point out as a reminder the document CH-A No. 386,454 which relates, not to the position recognition of linear displacement parts, but to that of pivoting parts such as typewriter keyboard keys. The recognition system is based on the occultation of a plurality of parallel beams by a coded screen, when a key is depressed. Such a system allows the recognition of the position of only one key at a time, and cannot be adapted to the recognition of the position of adjacent racks with linear displacement which would simultaneously occult the plurality of beams and would distort the recognition of the coded screens.